[Flac-dev] Bug in bitreader for short reads?

Gilles Boccon-Gibod bok at bok.net
Mon Jun 1 10:16:11 PDT 2009

It seems that the bitstream reader is not working as it should  
(verified in version 1.2.0 and 1.2.1).
The problem is as follows: if the data read by the read callback is  
not a multiple of 4, the bit reader will end up in a very bad state,  
where the bits_consumed field will grow without ever being reset to 0,  
and that causes everything to fail.
This is not a case that's encountered very often, because in general  
(like for example when reading from a file), the read callback will  
return as many bytes as were asked for, which happens to be a multiple  
of 4. But when reading from a network socket, or some type of  
buffering code, it can happen that reads are shorter than requested,  
and not always multiples of 4, and that produces decoding problems.
The problem is very easy to reproduce: in the read callback, always  
return 1 byte, regardless of how many bytes are asked for.
For example, in a typical read callback routine, one can do the  
(simplified) following and see the problem:

static FLAC__StreamDecoderReadStatus
FlacDecoder_ReadCallback(const FLAC__StreamDecoder* flac,
                          FLAC__byte                 buffer[],
                          size_t*                    bytes,
                          void*                      client_data)
     *bytes=1;   /* <--- this was added to always return at most one  
byte */

Any hints on how to get this fixed?

-- Gilles
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